An ability to measure the level and amount of a dry bulk solid or a liquid or fluid level in a container is often needed to know the rate of use or disappearance of material from the container so that rate of material use can be accurately established to enable the user to monitor and control the use of the material being withdrawn from the container and to know when to restock the container with the material after it is predicted to be depleted. Such a measuring device also alerts an operator to avoid overfilling the container and can monitor the refilling operation to ensure that the container is totally filled, but not overfilled.
This requirement is particularly important for agricultural livestock feed bins which are frequently discharged and recharged with feed. Many ways have been developed for making this type of measurement, but all suffer from the problem of being inherently complicated and expensive to employ for low-added value applications like livestock feed, and even though expensive, are often unreliable in the harsh usage environments often found in rural locations where feed bins are employed.
There are several examples in the prior art of bin level measuring devices. For example, Parsons (U.S. Pat. No. 3,629,946) shows a mechanical method which lowers a bell weight on a cable while measuring the length of cable deployed to bring the bell weight into contact with a material upper surface. This electro-mechanical system employs a complex system of pulleys and sheaves to make measurements which must use moving parts that are subject to wear and damage and which are insensitive to low-density and irregular material surfaces. Measurements are discontinuous and periodic, so continuous level changes are not easily recognized.
Baird (U.S. Pat. No. 3,912,954) teaches how the use of an improved narrow-beam ultrasonic transducer can be used to measure the depth of material in a silo by timing reflections of the sound waves from the material surface. This method has been found to be unsuccessful and unreliable in the harsh conditions in which feed bins are used. Further, the reflectivity of variable, low-density feed materials compared to other reflective surfaces such as metal bin supports and sidewalls and complex reflections from such surfaces make this method unreliable.
Greer (U.S. Pat. No. 4,043,199) shows the use of a tube suspended within a silo. The material in the silo compresses the tube and thereby engages and supports a portion of a chain suspended within the tube. The weight of unsupported chain is measured and related proportionately to the total weight of chain to determine the length of supported chain and thereby the depth of material in the silo. This system is complex, relies on a mechanical weight measurement employing electromechanical moving parts, and gives a discontinuous measure of depth which is not easily automated and subject to large errors owing to the approximate nature of the weighing mechanism. McGookin (U.S. Pat. No. 4,276,774) uses a similar method of measuring the unsupported weight of a suspended cable within a silo using a load cell and associated mechanisms, but without the use of the guard tube taught by Greer. This method suffers from the same shortcomings of Greer and is impractical for many applications including livestock feed silos.
A plurality of temperature sensors and associated circuitry are used by Beeston (U.S. Pat. No. 4,065,967) to measure a non-uniform difference in temperature within a silo as a method to detect a discontinuous change in temperature caused by a material change from air to granulated material content. This method is complex and expensive to implement, and level measurements are easily confused by temperature changes that are unrelated to material level, and by the very small changes in temperature caused by low density and low conductivity materials like livestock feed. The method is not continuous, but potentially can increase the frequency of depth measurements by simply increasing the number of temperature sensors.
The damping of a vibrating plate by material in contact with the plate is taught by Sogo (U.S. Pat. No. 4,107,994) as a method to measure material level in a silo. Sogo shows two vibrating plate detectors at the top and bottom of the silo that are used to detect the full and empty condition. Sogo describes the advantages of a vibrating plate device as well as alternatives to this method. A multiplicity of vibrating plate devices might be used together with the detection means taught by Sogo to measure changes in the level of material in a silo in a discontinuous way, but the cost of the system would increase proportionately and require numerous undesirable openings in the sidewall of the bin.
A reflected light-beam triangulation method is taught by Henry (U.S. Pat. No. 4,247,784), but such a system requires optics and photo-detectors that are quickly contaminated in the dusty conditions of livestock feed silos. This method also requires reflective material and complicated and expensive detection and computing equipment to convert small changes in angles of reflections into a depth measurement. Therefore, this method is impractical for inexpensive and reliable feed silo measurement.
Cournane (U.S. Pat. No. 4,807,471) shows a depth measuring method using electrical reflections from an air-material interface within a silo. An electrical wave is created by a frequency generator and conducted by electrical conductors suspended in the silo that convey an alternating frequency electrical wave, which is partially reflected by the air-material interface. The reflected wave is detected by a sophisticated detection circuit and microprocessor and related to depth. Cournane (U.S. Pat. No. 5,233,352) extends this method with improvements, but the method is complex, potentially expensive, and impractical for application for livestock feed silos. Similarly, Schreiner (U.S. Pat. No. 5,440,310) shows a complex microwave radar system for level measurement which suffers from the same deficiencies for common applications like livestock feed silos.
Salvo (U.S. Pat. No. 6,341,271) teaches an inventory method for material held in silos, but does not explain how the contents of silos are to be continuously and automatically measured in any practical way. Salmon (U.S. Pat. No. 6,608,491) also shows a complex and expensive method for powering and detecting the position of a plurality of paddle sensors arrayed within a silo. The sensors are deflected by the load of material above each paddle so a discontinuous approximate location of the surface of the material in the silo can be known.
Dirksen (U.S. Pat. No. 6,732,580) shows a load cell that monitors the weight of a suspended cable within a silo. The cable is fitted with a terminal weight to increase the sensitivity of the cable/weight assembly to changes in support provided by the material in the silo. The change in weight measured by the load cell is related to the level of contents in the silo. In practice, this system is found to be unreliable, requires the use of an expensive load cell, and otherwise is relatively insensitive to low-density feeds. The load cell is also very vulnerable to lightning and static electricity, which are maintenance risks for this concept.
Others have used load cells under the three or four legs of bulk bins to measure the contents of bulk bins by weight directly. However, load cells are expensive, very vulnerable to lightning and static electricity, and require frequent maintenance by skilled personnel and have therefore not been widely adopted by livestock producers. Such a method also cannot be easily used for silos that are not elevated on legs.
A commercial measuring system sold under the product name, Meritape, by JOWA Consilium US, Inc., Littleton, Mass. 01460-1431, http://www.consiliumus.com, utilizes a resistance element similar to that in the present invention, as described at http://www.consiliumus.com/Metritape.htm. An envelope covers a conductive and a resistive element which are brought into contact by the compression of the envelope by hydrostatic pressure. These envelopes of protective material are formed with an inherent shape and tension bias that is distorted by the hydrostatic pressure. The change in resistance arising from the compression-caused electrical contact is converted into a depth measurement for fluid materials.
The construction used for Meritape works well for relatively dense materials like liquids with high internal pressure, but is not capable of measuring the depth for relatively low-density, dry-bulk materials like livestock feed with low internal pressure because these shaped unsupported envelopes are limited in size by the inherent shape and tension bias that can be attained by unsupported envelopes. These envelopes are not substantially distorted at achievable shapes by dry, bulk materials and are insensitive to low internal pressures of such materials as livestock feed.